Coated abrasive articles generally comprise a backing to which a multiplicity of abrasive particles are bonded by a suitable bond system. A common type of bond system includes a make coat, a size coat, and optionally a supersize coat. The make coat includes a tough, resilient polymer binder that adheres the abrasive particles to the backing. The size coat, also including a tough resilient polymer binder that may be the same or different from the make coat binder, is applied over the make coat to reinforce the particles. The supersize coat, including one or more antiloading ingredients or perhaps grinding aids, may then be applied over the size coat if desired.
In a conventional manufacturing process, the ingredients that are used to form the make coat are dispersed or dissolved, as the case may be, in a sufficient amount of a solvent, which may be aqueous or nonaqueous, to provide the make coat formulation with a coatable viscosity. The fluid formulation is then coated onto the backing, after which the abrasive particles are applied to the make coat formulation. The make coat formulation is then dried to remove the solvent and at least partially cured. The ingredients that are used to form the size coat are also dispersed in a solvent, and the resultant fluid formulation is then applied over the make coat and abrasive particles, dried and cured. A similar technique is then used to apply the supersize coat over the size coat.
The conventional manufacturing process has some drawbacks, however, because all of the coating formulations are solvent-based. Typical make and size coat formulations may include 10 to 50 weight percent of solvent. Supersize coating formulations, in particular, require even more solvent in order to form useful coatings having the desired coating weight and viscosity. Solvents, however, can be expensive to purchase and/or to handle properly. Solvents also must be removed from the coatings, involving substantial drying costs in terms of capital equipment, energy costs, and cycle time. There are also further costs and environmental concerns associated with solvent recovery or disposal. Solvent-based coating formulations also typically require coating methods involving contact with underlying layers at the time of coating. Such contact can disrupt the orientation of the coated abrasive particles, adversely affecting abrading performance.
Not surprisingly, solventless manufacturing techniques have been investigated. One promising approach involves powder coating techniques in which a coating is formed by dry coating a powder of extremely fine, curable binder particles onto a suitable backing, melting the coated powder so that the particles fuse together to form a uniform melt layer, and then curing the melt layer to form a solid, thermoset, binder matrix. For example, PCT patent publication WO 97/25185 describes forming a binder for abrasive particles from dry powders. The dry powders comprise thermally curable phenolic resins that are dry coated onto a suitable backing. After coating, the particles are melted. Abrasive particles are then applied to the melted formulation. The melted formulation is then thermally cured to form a solid, make coat binder matrix. A size coat may be applied in the same way. Significantly, the make and size coats are formed without any solvent, and the size coat powder may be deposited without contacting, and hence disrupting, the underlying abrasive particles.
Notwithstanding the advantages offered by powder coating techniques described in PCT patent publication WO 97/25185, the powders described in this document incorporate resins that are thermally cured. The use of such resins poses substantial challenges during manufacture. Thermally cured resins generally tend to be highly viscous at reasonable processing temperatures, and thus are difficult to get to flow well. This makes it somewhat challenging to cause the binder particles to melt and fuse together in a uniform manner. The thermally curable resins also typically require relatively high temperatures to achieve curing. This limits the kinds of materials that can be incorporated into an abrasive article. In particular, many kinds of otherwise desirable backing materials could be damaged or degraded upon exposure to the temperatures required for curing. It is also difficult to control the start and rate of thermal curing. Generally, thermal curing begins as soon as heat is applied to melt the powder particles. As a consequence, the cure reaction may proceed too far before the powder particles are adequately fused. Further, the resultant bond between the cured binder and the adhesive particles may end up being weaker than is desired.
Accordingly, there is still a need for a solventless manufacturing technique for making abrasive articles that avoids disrupting abrasive particle orientation as the various component layers of the abrasive bond system are formed.